arraylist.size（）java空指针异常

Question

我已经阅读了其他答案，试图找到解决方案，但没有明确解释问题。

为什么以下代码块会为currArraySize创建一个空指针异常？

Private Sub TextBox1_MouseDown(ByVal Button As Integer, ByVal Shift As 
    Integer, ByVal X As Single, ByVal Y As Single)

    initialx = X
    initialy = Y
    xmove = True
End Sub

Private Sub TextBox1_MouseMove(ByVal Button As Integer, ByVal Shift As 
    Integer, ByVal X As Single, ByVal Y As Single)
    Dim newx, newy As Single

    If xmove = True Then
        newx = Math.Abs(X - initialx)
        newy = Math.Abs(Y - initialy)
        TextBox1.Width = TextBox1.Width + newx
    End If
End Sub